Analytical processes such as mass spectrometry produce information acquired by processing charged species of sample analyte materials. The charged species are produced by ionizing the sample materials. Certain popular techniques for ionizing sample materials utilize a beam of energetic electrons. These techniques include electron ionization or impact (EI) and chemical ionization (CI). EI entails ionizing sample materials by transferring energy from the electrons to sample materials. CI entails ionizing a reagent gas by transferring energy from the electrons, and then ionizing sample materials by reactions with the ionized reagent gas. An electron source that includes a heated thermionic electrode, typically in the form of a filament, may be employed to produce the electron beam. Ion guiding techniques are employed to control the electron beam and focus the beam into an ionization chamber containing the typically gas-phase sample material. The chamber may be external to an ion processing device such as an ion trap or may be within the ion processing device.
The operation of an ion processing device, particularly a pulsed-operation device such as an ion trap, includes periods during which ionization is performed and periods during which ionization is not desired. Electrical circuitry communicates with the thermionic electrode as well as other electrodes utilized to control the electron beam. Depending on the configuration, the electron beam may be gated ON and OFF, adjusted between a HIGH state in which the electrons have sufficient energy to ionize sample material and a LOW state in which their energy is not sufficient to do so, or run continuously with the direction of the beam alternating toward and away from the ionization chamber. In all such configurations, control over and consistency of the electron emission current are desirable. Accordingly, the electrical circuitry associated with the electron source is typically capable of regulating the heating current passing through the thermionic electrode to maintain the total electron current at some constant, predetermined value.
Unfortunately, various components of the electron source are prone to becoming fouled with contaminating material, particularly when operating the CI mode of ionization. The contaminating material may be electrically conductive and thus may engender leakage currents. Leakage currents can cause several problems, including reducing the total emission current intended to be produced by the electron source, increasing electrical and/or photon noise, impairing the function of the emission current regulator and, more generally, impairing the accuracy and effectiveness of the associated ionization source.
In view of the foregoing, it would be advantageous to provide methods and apparatus that prevent, suppress, reduce, or eliminate leakage currents in electron sources, particular electron sources employed in conjunction with ion sources.